Ultrasonic gas flow measurement method and apparatus

ABSTRACT

A transit time gas flow meter comprising a pair of transducers mounted at an oblique angle to gas flow in a duct for measuring the transit time of an ultrasonic pulse in both directions between the transducers and computer determines the difference between transit times as a measure of the velocity of a gas stream flowing in the duct. Each transducer is carried in a mounting assembly which is comprised of a U-shaped frame member having a base and leg members and an ultrasonic reflection surface formed on the base of U-shaped frame member. A transducer is mounted between said legs and at a predetermined oblique angle to said reflection surface. A reference target carried on the U-shaped frame member is positioned at the edge of said gas and the ultrasonic transit time to each said reference target is subtracted by the computer means from the transit times from each of transducer to the other transducer, respectively, to zero flow calibrate transit time flow meter and to improve the accuracy measurements.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION:

The invention relates to gas velocity or flow monitors the transmit timeof an ultrasonic pulse between an ultrasonic sender and receiver changesaccording to the velocity of the gaseous medium flowing between thesender and receiver.

Typically, a pair of ultrasonic transducers is mounted at an obliqueangle to gas flow in a duct and on a common ultrasonic axis. Thetransducers operate as both senders and receivers and transit timemeasurements are made with and against the direction of flow so thatfactors affecting sonic velocity such as temperature, pressure, etc. areeliminated and, only the transit times, when the ultrasonic pulses aretransmitted in the direction of flow and the sonic transit time againstthe direction of flow, are measured to determine gas velocity. These aredenoted transit time flow meters as opposed to ultrasonic measurementswhich are based on Doppler effect.

The object of the present invention is to provide an improved transittime gas flow meter which is more accurate, easier to install and oflower cost.

In particular, an object of the invention is to provide a transit timeultrasonic measuring system for measuring gas flow which may becalibrated for zero flow bias. Further, a feature of the invention isthat the up stream and down stream ultrasonic transducer reflectorassemblies incorporate acoustically reflective surfaces which are easilymounted in acoustic view of each other. The upstream and downstreamtransducer assemblies incorporate a rectangular housing which includes amounting bracket for orienting a transducer, particularly a low-costnarrow beam electrostatic transducer of the Polaroid®-type which arepre-oriented relative to a Teflon® coated reflective surface andincludes a reference wire target which is positioned at the edge of theflow path of the gas being monitored. While the reference targets couldadvantageously be located along the acoustic axis of the reflectivesurfaces and the transducers, it preferably is located at the edge ofthe respective reflectors so that it is out of the path of any sidelobes of the transducer and, even though it may not be in the rightplace, since it is a known distance, the signal microprocessor caneasily attend to the fact that it is not on the acoustic axis. Thereference targets provide for increased accuracy in flow measurement andzero flow calibration of the instrument.

DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the inventionwill become more apparent when considered with the followingspecification and accompanying drawings wherein:

FIGS. 1a and 1b are illustrations of prior art transit time monitors,

FIG. 2 is a schematic diagram illustrating the present invention,

FIG. 3 is a sectional view of one transducer arrangement mounted in anexisting flue-gas attachment,

FIGS. 4-10 are views of the transducer and reflector assemblyincorporating the invention,

FIG. 11 is a block diagram of the electronic circuitry incorporated inthe invention, and

FIG. 12 is a simplified graph showing the transmission and receptionsignals of the two transducers.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the prior art illustrated in FIG. 1a, ultrasonictransducers 10 and 11 are mounted on a common canted acoustic axis inthe sidewalls 12 and 13 of a gas flow duct 14 and have a distance Lbetween the transducers defining the path through the moving fluidmedium whose velocity V is to be measured. Evaluator 15 solves theequation: ##EQU1## where K is a constant that results from the mountinggeometry and used to calibrate the evaluator, T_(v) is the ultrasonicpulse transit time in the flow direction, and T_(r) is the sonic transittime against flow. The signals from the evaluator are supplied to arecorder 16 which may include a strip chart recorder and to a computer17 which receives measured emission values of dust, SO₂, NO, CO₂, O₂,etc.) and computes the quantities of dust, SO₂, NO, etc. which areflowing in the flue gas duct 14. A source of purge gas is supplied toeach of the transducer assemblies 10 and 11 to provide protectionagainst contamination.

In the prior art device shown in FIG. 1b (which is FIG. 10 of LynnworthU.S. Pat. No. 4,004,461), a holder 129 mounts a transducer 131 whoseaxis is parallel to the opposing transducer on the opposite side (notshown in FIG. 1b) and the direction of the transducers are in a radialdirection in first bore holes 119. Each transducer assembly includes anoblique second bore hole 120 which is machined with its axis at 22.5degrees to the radial direction so that the radially incident beam ofthe transducers are reflected along an inclined path at 45 degrees tothe axis of the thick walled pipe 119 and is reflected off of a flatreflecting face 121. The transducers are mounted in separate portionswhich have flanges 122 joined together by bolt means and the spacebetween the gaps has a series of gaskets 123 interposed between theflanges and these gaskets are formed of low acoustic impedance materialto provide an acoustic mis-match for the conduit walls 119. One assertedadvantage of the assembly shown in FIG. 1b is that the use of separatepipe sections for the transmitter and receiver transducers permitscertain machining operations be performed and permits the transducerholders to be mounted in a radial orientation and yet provide an obliqueinterrogation path at an angle which is substantially immune to soundspeed changes in the fluid.

THE PRESENT INVENTION

Referring now to the diagrammatic illustration in FIG. 2, a pair ofsubstantially identical transducer/reflector assemblies 30 and 31 havebeen dimensioned to reflect the 3-4-5 right triangle relationship shownin the drawing so as to permit relative ease of alignment andinstallation. Each transducer/reflector/reference target assemblyinclude a transducer T₁ which, in a preferred embodiment, are narrowbeam Polaroid® electrostatic transducers but which may be piezoelectrictransducers, and the like, and are mounted at an angle (α) relative to ahorizontal and thus has its acoustic axis AA oriented to reflect off ofa reflecting surface R₁ which is perpendicular relative to the directionof flow of gases denoted by arrow AG and, with reference to thereflective surface R2 associated with the transducer T₂ so as to satisfythe 3-4-5 right triangular relationship for the simplified installationprocess. Reference targets RT₁ and RT₂ may be located along the acousticaxis and at the edge of the flow path boundary for the gases flowing induct D at alternative reflector reference target positions RTA.sub. 1and RTA₂. Since these positions (RTA₁ and RTA₂) may be impinged by aside lobe from each of their respective transducers T₁ and T₂, apreferred position for these reference targets RT₁ and RT₂ are

at the edge of the reflectors R₁ and R₂ and just at the edge of the flowstream of flowing gas in duct D. (These wire reference targets may evenbe slightly in the flow path beyond the edges of the duct D.) In thisposition, the improved accuracy and calibration for zero flow conditionsutilizing the targets RT₁ and RT₂ can be accomplished without concernfor affects of side lobe signals or pulse energy emanating from thetransducers T₁ and T₂.

It will be noted that the up and downstream acoustically reflectivesurfaces R₂ and R₁, respectively, are in optimum unobstructed ultrasonicview of each other because of the three, four, five right triangularrelationship discussed above.

Referring now to FIG. 3, the invention is shown as applied to a duct D,a circular tube 36 carrying the transducer, reflector and referencetarget assembly 37 is supplied with purge gas from a pancake or muffinfan 38 which has a plenum chamber 39 having an air filter 40 forreceiving and filtering ambient air or other purge gas, if desired. Itwill be appreciated that while pipe 36 is shown as being circular, itcould be rectangular and conforming to the rectangular shape of thetransducer and reflector assembly 37.

The transducer and reflector assembly is shown in detail in FIGS. 4-10with FIG. 4 being a side elevational view and FIG. 5 being an end viewlooking inwardly through the end of pipe 36. Dimensions shown are purelyexemplary.

The transducer and reflector assembly 37 includes a U-shaped frame 38having sidewalls or legs 39 and 40 joined by a reflector wall or base41, and a transducer mounting plate strap 42 which is welded betweensidewalls or legs 39 and 40. Sloping edges 43, 44 define the angularorientation of the acoustic axis of the transducer T relative to thereflecting surface 41. As shown in FIGS. 4 and 6, a transducer mountingplate 45 has an angular member 46 which is bolted through bolt holes 47,48 in cross member 42. Stand-off posts 49 space the TR switchpreamplifier printed circuit board from the transducer T1. U-shapedframe 38 is preferably made of stainless-steel and at least thereflecting surface 41 is coated with a non-stick polymer such as Teflon®(PTFE) and the interior side surfaces 39-40 may likewise be coated withthe same non-stick material to minimize contamination thereof and reducemaintenance.

It will be appreciated that high velocity flue and stack gases, forexample, will be flowing in the duct D and therefore in addition topurge gas flow will create negative pressure in the pipe 36 causing outflow but that there will be eddys and turbulences in the pipe so thateven with this negative pressure and the purge gas, there still may beparticles of materials which could stick to these surfaces and, thenon-stick coating of Teflon® (PTFE) minimizes the sticking.

Each of the transducer and reflector assemblies includes or supports areference target RT which is used in accordance with the invention inorder to enhance accuracy and establish zero flow calibration. Asdiscussed above, these reference targets are positioned at the lateralboundaries or edges of the fluid flowing in the duct D and optionallyalong the acoustic axis thereof. However, since the transducers T₁ andT₂ are mounted at angles relative to the reflector surface 41,positioning of these reference target reflectors on the acoustic axis,as for example, at position designated RTA₁ and RTA₂ in FIG. 2, sidelobes may be impinged upon these reference targets and hence, in apreferred practice of the invention, they are positioned at the end ofthe reflectors R₁ and R₂ and just at the edge of the flow stream of thegas flowing in duct D. In this situation, a slight proportionalitycorrection is entered when the computation is made for zero flowcalibration and for the ultrasonic transit times between thetransducers.

These reference targets RT are comprised of stainless-steel wires orrods which have preferably been coated with non-stick material such asTeflon® (PTFE) and preferably in the same coating operation as done withthe ultrasonic reflectors surfaces R₁ and R₂. These wires or rods aremounted in small apertures or bores at the ends of the sidewalls 39 and40, respectively.

At this point it should be noted that these reflector reference targetsdo not perform the same function as in those cases where a referencetarget is utilized for purposes of establishing temperature compensationand the like. In the present case, since there is a transmission of theultrasonic signal in the direction of flow and in the oppositedirection, the equation eliminates factors affecting sonic velocity suchat temperature, pressure, and the like. In the present invention thereference targets RT are used to provide zero flow calibration and toimprove the accuracy of the measurements, particularly for smallerdimensioned flow ducts D.

Referring now to the block diagram of FIG. 11 and the graph of FIG. 12,transducers T₁ and T₂ are alternately activated with about a 50 kHzsignal pulse via transmit/receive and driver networks 50 and 51 which,in turn, are controlled by control signals on channels 53 by computer(microprocessor) controller 52. First, a signal is transmitted to getthe echo time reference T₁,1 and T₂,2 to the reference targets RT₁ andRT₂ and this is done by transmitting a pulse or ping and, at the sametime, this signal is supplied to start the timer 55. Also, a signal online 56 from computer controller 52 is applied to range gate 58 so as toopen the range gate quickly so that the signals from the referencetargets RT₁ (or RT₂) via the preamplifiers PA₁ and PA₂ can pass. Becauseof the relative shortness of the distances, the reflection from thereference targets comes back very quickly. The signals from the rangegate 58 are supplied to automatic gain control circuit 60 which has beenpreviously set by signals from computer controller 52 to the correctlevel so that the zero crossing detector 62 works from the right phaseof the signal coming back and the signal from the zero crossing detector62 is supplied to the timer 55 to stop the timer. This is time to andfrom the reference target and is divided up by 2 to get the timemeasurement to the reference target. It should be kept in mind that halfof the time is on one side e.g., associated with one reference targetand transducer, and the other half is on the other side e.g., associatedwith the other reference target and transducer, so the average of thesetwo time intervals T₁,1 and T₂,2 (the time intervals to the tworeference targets RT₁ and RT₂) are then subtracted from the total time,either the upstream traverse or the downstream traverse (against theflow or with the flow), for enhancement of accuracy.

The AGC circuit 60 supplies the signal to the peak detector 64 whichindicates whether or not there is a detection. If there is no detection,the computer controller 52 increases the gain or if there is a detectionthen it may decrease the gain and the computer is programmed to allow itto get up gain fast and to stabilize. It should be noted that there arefour different gain numbers that are stored, one for each of thetransducers and one for each of the reference targets. In the preferredembodiment, transducer T₁ and T₂ are not operated simultaneously butalternate in sequence so that the circuits from the range gate circuitryto the timer can operate in multiplex fashion. However, it will beappreciated that a separate circuitry can be utilized for each of thetransducers which are controlled by a common computer or data processor.The computer controls the application of the control signals to therange gates in this case. It can also perform the timer functions aswell, if desired.

The time is measured to each of the reference targets on each transducerand those times are compared. This is essentially a comparison of thephase response of the two transducers and for extreme accuracy and forzero flow bias, the output of the flow meter when there is no flow is afunction of the difference in the phase response of the two receivingsystems. This means that when the electronics are time shared ormultiplexed, the receiving electronics are time shared as much aspossible to get any imbalance in the phase response out of the system.However, there is always left the transducer and the transmit/receivecircuit and the preamplifier associated therewith. Thus, to that extent,the phase response of one transducer amplifier combination is differentfrom the other there will be an equivalent ΔT or time difference whenthere is no flow. The reference targets therefore allow thedetermination of this ΔT while the machine is operating and allows thecomputer to assess the difference in phase responses when thetemperatures are the same. That is, when the purge temperatures are thesame and which they will be approximately. Alternatively, purge airtemperatures may be measured and used to correct ΔT . This permits theapparatus to comply with the EPA requirements that the apparatus have azero flow calibration feature. With the references targets of thisinvention, the apparatus is able to give a true reading of the relativephase stability between the two systems of the electronics that wereused differently in the upstream and downstream phases and it allows thesystem to test itself.

In the time graph shown in FIG. 12, the initial ping of transducer T₁ isillustrated at P₁ and the reflection or echo by the reference target RT₁as received by transducer T₁ is indicated as S₁. The next signalreceived by transducer T₂ is the signal S₂ from transducer T₁. The rangegate has been opened a to allow each signal to pass. The main transducerping P₂ is indicated for transducer T₂ and the range gate is opened toallow the reference target RT₂ echo S₃ to be received by transducer T₂.The next signal time the range gate is opened is to allow the signal S₄received by transducer T₁ to pass. The computer controller 52 performsthe calculations discussed above, and at the same time, subtractsone-half of the time interval from T₁ to RT₁ and one-half of the timeinterval from T₂ to RT₂ to arrive at T_(r) and T_(v) transit times andperform the flow velocity calculations.

This use of the reference targets RT₁ and RT₂ in the calculation of thetransit times becomes more significant the smaller the pipe or duct D sothat the smaller the pipe the larger or more significant becomes themeasurement to the reference targets RT₁ and RT₂ and their use inarriving at a more accurate measurement of the velocity of the flow ofgases in duct D.

The zero flow calibration should derive the zero flow time differencefrom the times to the reference targets RT₁ and RT₂. However,temperature compensation would be required for best accuracy inmeasurements to the reference targets and thus a temperature measurementcapability could be incorporated into the system, but not necessarily tothe practice of the invention.

While there has been shown and described preferred embodiments of theinvention, it will be appreciated embodiments and adaptations of theinvention will become readily apparent to those skilled in the art andis intended to encompass such obvious modifications and adaptationswithin the spirit and scope of the claims appended hereto.

What is claimed is:
 1. In a transit time gas flow meter comprising apair of transducers mounted at an oblique angle to gas stream flow in aduct for measuring the transit time of an ultrasonic pulse in bothdirections between said transducers and computer means for determiningthe difference between said transit times as a measure of the velocityof a gas stream flowing in said duct, the improvement comprising,amounting assembly for each of said transducers, each said mountingassembly including an elongated U-shaped frame member having a base andleg members, an ultrasonic reflection surface formed on said flat basemember, means for mounting one of said pair of transducers between saidsidewall members and at a predetermined angle to said ultrasonicreflection surface formed on said base member, a zero flow calibrationtarget, means forming upstream and downstream openings in said duct, andmeans for supporting and positioning a mounting assembly in each one ofsaid openings, respectively, on said duct with said zero flowcalibration target at the boundary edge of said gas stream flowing insaid duct and the sonic path between said ultrasonic reflection surfacedefining said oblique angle.
 2. The transit time gas flow meter definedin claim 1 wherein the ultrasonic transit time to each said zero flowcalibration target is subtracted by said computer means from the transittimes from each of said transducers to the other transducer,respectively, to zero flow calibrate said transit time flow meter. 3.The transit time gas flow meter defined in claim 1 wherein said zeroflow calibration target is positioned at the edge of said ultrasonicreflection surface.
 4. The transit time gas flow meter defined in claim2 wherein said zero flow calibration target is positioned on theacoustic axis of said ultrasonic reflection surface.
 5. The transit timegas flow meter defined in any one of claims 2-4 and 1 wherein each saidultrasonic reflection surface is coated with a non-stick PTFE polymer.6. The transit time gas flow meter defined in any one of claims 2, 3 or4 wherein each said zero flow calibration target is coated with anon-stick PTFE polymer.
 7. In a transit time gas flow meter comprising apar of transducer means mounted at an oblique angle to gas stream flowin a duct for measuring the transit time of an ultrasonic pulse in bothdirections between said transducer means and computer means fordetermining the difference between said transit times as a measure ofthe velocity of a gas stream flowing in said duct, the improvementcomprising,a mounting assembly, a transducer carried by the mountingassembly, each mounting assembly comprising a frame member havingsidewalls and a flat base and a transducer support member, an ultrasonicreflection surface formed on said base, means for securing saidtransducer and said transducer support member at a predetermined angleto said ultrasonic reflection surface, means forming upstream anddownstream openings in said duct at opposing sides thereof, a zero flowcalibration target positioned at the boundary edge of the gas streamflowing in said duct means for mounting and positioning each of saidframe members in said openings at opposing edges, respectively, of saidgas stream flowing in said duct so that the said ultrasonic reflectionsurfaces are aligned along said oblique angle.
 8. The transit time gasflow meter defined in claim 7 wherein said predetermined acoustic axisis at an angle to the direction of flow in said duct such that saidacoustic axis is aligned with the hypotenuse of a 3, 4, 5 right triangleto simplify and facilitate installation thereof on said duct.
 9. Thetransit time gas flow meter defined in claim 7 wherein each of saidmounting assemblies include a zero flow calibration target which is atthe edge of said gas stream when said mounting assembly is mounted onsaid duct.
 10. The transit time gas flow meter defined in claim 9wherein said zero flow calibration target is substantially on saidacoustic axis.
 11. The transit time gas flow meter defined in claim 7wherein said zero flow calibration target is positioned at the edge ofsaid reflector surface.
 12. The transit time gas flow meter defined inany of claims 7, 8, 9 or 11 wherein said zero flow calibration target isa wire coated with PTFE polymer.
 13. In a transit time gas flow metercomprising a pair of transducers, each said transducer being spaced fromthe boundary edges and on opposite sides of, and at an oblique angle toflow of a gas stream in a duct, respectively, means for measuring thetransit time of an ultrasonic pulse in both directions, respectively,between said transducers and computer means connected to saidtransducers for determining the difference between said transit times asa measure of the velocity of gas stream flowing in said duct, said ductdefining the boundary edges of said gas stream, the improvementcomprising:a zero flow calibration target for each of said transducer,and means positioning one of said zero flow calibration targets at aboundary edge, respectively, of said gas stream and a predetermineddistance from its associated transducer, said computer means beingprogrammed to subtract the sonic transit times to said zero flowcalibration targets from their respective transducers from the sonictransit times between said transducers.
 14. The transit time gas flowmeter defined in claim 13 wherein each said zero flow calibration targetis located on the acoustic axis between transducers.
 15. The transittime gas flow meter defined in claim 13 including a sonic reflector foreach of said transducers, each sonic reflector being located between itsrespective transducer and said boundary edge of said gas stream flowingin said cut means orienting the acoustic axis of said transducers at anpredetermined angle relative to said sonic reflectors, respectively, anda path defining the acoustic axis between said reflectors which is atsaid oblique angle to the flow of a gas stream in said duct.
 16. In atransit time gas flow meter comprising a pair of transducers, saidtransducers being spaced from the boundary edges and on opposite sidesof, and at an oblique angle to flow of a gas stream in a duct, means formeasuring the transit time of an ultrasonic pulse in both directions,respectively, between said transducers and computer means connected tosaid transducers for determining the difference between said transittimes as a measure of the velocity of gas stream flowing in said duct,said duct defining the boundary edges of said gas stream, theimprovement comprising:a zero flow calibration target for each of saidtransducer, and means positioning one of said zero flow calibrationtargets at a boundary edge, respectively, of said gas stream and apredetermined distance from its associated transducer, said computermeans being programmed to subtract the sonic transit times to said zeroflow calibration targets from their respective transducers from thesonic transit times between said transducers, a sonic reflector for eachof said transducers, each sonic reflector being located between itsrespective transducers and said boundary edge of said gas stream flowingin said duct, means orienting the acoustic axis of said transducer at apredetermined angle relative to said sonic reflectors, respectively, anda path defining the acoustic axis between said reflectors which is atsaid oblique angle to the flow of a gas stream in said duct wherein eachsaid reference target is located at an end of an associated sonicreflector surface.
 17. The transit time flow meter defined in claim 16wherein said zero flow calibration target is located on the acousticaxis between said reflectors.
 18. The transit time flow meter defined inclaim 16 wherein said reference target is at the edge of its respectivesonic reflector.